There has long been an appreciation of the importance of osteoporosis in women, and decades of research have yielded a strong foundation of mechanistic and practical knowledge that underlies clinical decision-making. On the other hand, very little information is available to direct the management of osteoporosis in men. Data are gradually emerging, but are derived primarily from cross sectional studies and remain fragmentary. The motivation for this proposal is to provide the essential groundwork for rational clinical and public health decisions concerning osteoporosis in men, and to direct future studies of pathophysiology and therapeutics. UCSF will serve as the Coordinating Center for the six clinical centers. These centers will recruit approximately 5700 older men, characterize selected variables of probable importance to skeletal health, and to identify the major determinants of fracture risk over a 3+ year follow-up period. The proposed study will adopt measures that are in many ways identical to those utilized in the Study of Osteoporotic Fractures, an ongoing, large, longitudinal observational study of the determinants of fracture risk in women conducted by the current investigative team and led by the UCSF CC). The UCSF Coordinating Center has had extensive experience in coordinating a number of large NIH and industry sponsored multicenter studies of osteoporosis that are directly relevant to the proposed study in men. UCSF-coordinated studies include the Study of Osteoporotic Fractures, the Fracture Intervention Trial the Fracture Intervention Trial Extension (FLEX), the Health, Aging and Body Changes (Health ABC) study, the Multiple Outcomes of Raloxifene Evaluation Trial (MORE), and osteoporosis-related functions of the Women's Health Initiative (WHI). As we have for other studies, we will provide comprehensive operations manuals, clear scannable data collection instruments, expert training, convenient communications systems, state of the art data management system, experienced management of all aspects of densitometry, skeletal radiology, and QCT assessments and quality control. The end product will be well documented distributed data sets for analysis and production of landmark papers about skeletal health in men.